Most electric motors have traditionally been provided with both stator and rotor windings, even though in many induction motors the latter may be simplified to a "squirrel cage", and rely upon either conduction through commutators or slip rings, or upon induction, to energize the rotor. Induction motors normally require an alternating supply for their operation, and are not in general well adapted to variable speed operation since their optimum operating speed is intimately related to the velocity of the rotating field generated by the alternating supply. Direct current motors on the other hand require some form of commutative switching of the supply to the rotor to provide continuous rotation, and such commutators are expensive to build and maintain, as well as a source of undesirable broadband electrical interference. Control of such motors where accurate speeds or displacement control is required remains complex and difficult.
As a result, attention has been given, for a wide range of potential applications ranging from motors for consumer electronic equipment to large appliance, traction and industrial motors, to motors of the reluctance type in which the current through stator windings is switched, usually in modern designs by solid state devices, so as to produce a changing electromagnetic field which will result in progressive angular movement of poles of a stator as it seeks a minimum reluctance position within the field. This movement may be in the form of discrete steps, individually controlled, as in a stepper motor, or the movement of the rotor may be sensed by some suitable means to switch the current through the stator windings so as to provide a free running mode in which successive steps or impulses run together to provide continuous rotation. Regardless of the mode employed, the inductance of the windings provides difficulties as they are progressively switched, since it limits the rate of increase of the current upon energization and the rate at which magnetic energy can be dispersed when no longer required, particularly if excessive potentials are not be induced in the windings.
One widely used approach to the second of the above problems has been to utilize so-called "free-wheeling" diodes connected across the various windings. When external current to a winding is interrupted, the diode provides an alternative path for the current induced in the winding by the collapsing magnetic field, and the current thus recirculates until the field is fully collapsed, giving a slow fall in current. The rate of collapse can be increased by incorporating a resistive element in the circuit, but this reduces efficiency. Such a resistive element can also be used to assist rapid build up of the field, by acting as a current limiting device which permits application of higher energization potentials than would otherwise be possible. In many actual or potential applications of such motors, efficient operation and high torque over a wide range of speeds is required, and to attain these objectives it is necessary to achieve rapid current rise and fall times in the windings without unnecessary dissipation of energy as heat so that the fields of the stator and rotor can be maintained in optimum relationship. If rise and fall times are too slow, there will either be overlap with different windings producing opposing fields at some stages in the cycle, or the speed and/or torque obtainable will be limited.
One approach to the problem of obtaining rapid fall times has been to regenerate current from the stator windings to the supply. Thus in U.S. Pat. No. 4,229,685 issued to Meier, the freewheeling diodes are supplemented by diodes which divert current through a regulator circuit and back to the supply thus recovering the energy stored by the field generated by a winding following deenergization of the latter whilst assisting in rapid collapse of the field. In order to promote rapid build up of the magnetic field, however, Meier utilizes a chopping current regulator to limit current through the motor winding, which also serves to select a particular winding, together with a secondary switch which takes the freewheeling diode out of circuit except when that winding is energized. Such a system requires that the supply potential to the motor be high enough to provide the desired rate of current build up in the windings, and also requires the use of chopping regulators capable of sustaining the supply potential. The Meier patent refers to a stepping motor which can be operated in free running mode. A somewhat similar arrangement is described in U.S. Pat. No. 4,459,519 issued to Erdman. This relates to a motor with a permanent magnet rotor apparently primarily intended for refrigeration systems, and whilst a different system is used for regulating the current in the windings, the rate of current build up is still limited by the supply potential. Yet further similar arrangements as applied to various configurations of motor having magnetic rotors of both homopolar and heteropolar constructions are described in U.S. Pat. No. 3,826,966 issued to Nagasaka et al. Yet a further arrangement operating upon this principle is shown in U.S. Pat. No. 3,748,554 issued to McDonald.
A further problem which frequently arises in the design of brushless DC motors is that of turning off the switching semiconductors utilized to provide control of the current supplied to the field windings. The most readily available and economical semiconductors for the purpose are thyristors which have a controlled turn on ability but usually can only be turned off by reducing the current through the device to near zero. Furthermore, when turn off is achieved, stored energy in the inductive circuits being controlled can give rise to high potential spikes which can destroy the semiconductors if not properly controlled. For this reason commutation circuits have been developed for use in such applications which are essentially of ring counter configuration in that the turn on of the device controlling one winding is utilized to discharge one plate of a capacitor connected to the supply to the previously turned on device so as momentarily to divert the current to that device to the other plate of the capacitor and thus interrupt the current flow through the device for long enough that it switches off. Once it is switched off, recharging of the capacitor occurs, thus taking up some of the energy from the collapsing field of the associated winding.
Although the capacitors used in such circuits can contribute to the transfer of surplus energy from one winding to the next, this is not their primary purpose, and the arrangement is only useful in cases where the supply to a following winding can be turned on before that to a previous winding is terminated. Examples of such arrangements may be found in U.S. Pat. Nos. 3,611,081 issued to Watson, and 4,445,077 issued to Kirschner.
In U.S. Pat. No. 3,444,447 issued to Newell, an arrangement is described for improving the rise and fall times of currents in the windings of a step motor. Firstly, the supply is utilized to charge capacitors associated with control circuits for each winding, the circuit being arranged and the capacitor being switched so that its charge potential is added to the supply potential when the associated winding is energized, thus initially boosting the supply potential and improving the current rise time. Additionally, as described with reference to FIGS. 7 to 9, an arrangement using diodes and/or autotransformers is utilized to transfer energy from the collapsing field of a winding which has just been turned off to boost the potential applied to a winding that has just been turned on, thus improving both rise and fall times and improving efficiency. The first of the techniques disclosed by Newell provides a degree of boost which is substantially constant regardless of operating conditions, whilst the second technique is applicable only where the turning on of one winding is simultaneous with the turning off of another.
In U.S. Pat. No. 3,486,096, issued to Van Cleave, windings of a stepper motor are transformer coupled in pairs, and the switching means for each winding is operative to block current flow in a forward direction only. One or more diodes are placed in series with a D.C. supply so that current can flow from the supply in a forward direction only, and unswitched ends of the windings, or pairs of them, are connected to a capacitor or capacitors whose other plates are grounded. When forward current through a winding is interrupted, a current in the reverse direction is induced in the winding coupled thereto, and charges the associated capacitors to a high potential, whilst the field produced by the original winding rapidly collapses. When a switching device again permits forward current through a winding connected to the capacitor, the high potential carge on the capacitors assists rapid current build-up in that winding. The primary purpose of the arrangement is to speed up operation and protect the switching device; efficiency is evidently not a concern since resistors are placed in series with the supply to limit current. Moreover, the device is applicable only to motors having a suitable winding arrangement so that transformer action may be utilized to reverse the direction of current flow in the windings during energy recovery.
A group of related U.S. Pat. Nos. 3,560,817 and 3,560,818 issued to Amato, 3,560,820, 3,697,839 and 3,714,533 issued to Unnewehr, and 3,697,840 issued to Koch, and all assigned to Ford Motor Company, relate to various configurations of control circuits for reluctance type motors, in each of which a tuned circuit comprising capacitors and inductors (which may be or comprise the motor winding) are used in conjunction with solid state switching elements, utilizing resonance effects to increase the effective potentials available to provide fast rise and fall times, and to reverse the polarity of charge received from the circuit when a primary supply is cut off. Although there are differences between the arrangements described in these various Ford patents their general principle of operation relies on drawing current from the primary supply in pulses of approximately half-sine wave form. Since the period of the pulses is set at a substantially constant magnitude by the reactive components in the circuit, provision for different motor speeds is provided by varying the number of pulses delivered during each energization phase of a winding, substantial continuity of current flow in the winding between pulses being obtained both by freewheeling effects and by charge reversal and re-application of energy recovered during field collapse. In some of the arrangements, the circuit is operated so as to build up potential on a capacitor to a level much greater than the supply, which potential is applied so as to augment the magnitude of the current pulses from the supply. In the Unnewehr U.S. Pat. No. 3,714,533, it is disclosed that surplus energy from this capacitor may be tapped off by suitably timed firing of an SCR and returned to the supply if not required to drive the motor. Various methods for controlling the various motors disclosed are discussed, in general involving fairly complex control of the firing sequence of the several controlled rectifiers associated with each winding. In each case, it appears that operation requires an inductor in series with the supply additional to the motor winding, and that the operating parameters of the circuit are critically dependent upon the value of this inductor and also those of an energy storage capacitor. These same elements also limit the rate at which energy can be drawn from the supply, since the resonant characteristics of the load limit both the periods over which current can be drawn from the supply and the rate of supply current rise and fall.
U.S. Pat. No. 4,025,831 discloses a motor having in one embodiment plural stator windings and a permanent magnet homopolar rotor in a physical arrangement somewhat resembling the physical arrangement of the preferred embodiment of the motor described hereinbelow. The control system of the motor is however quite different, as is the mode of operation, no special provision being made for improving current rise, and fall times in the stator windings, or for recovering energy from collapsing stator fields.
An object of the present invention is to provide a motor of the general class discussed in which rapid rise and fall of winding current can be obtained at timings appropriate to ensure effective development of motor torque over a wide range of motor speeds, without the necessity for expedients which are wasteful of energy (such as added series resistance), without unduly restricting the rate at which energy can be drawn from the supply to meet torque demands, and without the necessity for highly sophisticated control means for matching the motor characteristics to load requirements.
According to the invention, an electric motor, of the type having a stator with multiple sequentially energizable phase windings and a rotor magnetized to seek a minimum reluctance position with a progressively moving electromagnetic field produce by said phase windings, first controlled switching means in series relative to a D.C. supply with each phase winding, and means to control said first switching means to produce said progressively moving electromagnetic field has (a) a charge storage capacitor associated with each such phase winding, with one terminal of said capacitor having a low impedance path to the supply, and the other terminal having alternative connections to opposite ends of the winding, the first such connection being established by diode means to that end of the winding connected to the first switching means, the diode means being oriented to permit passage of forward current continuing in said winding after turn-off of the switching means, and the second such connection to the other end of the winding being established by second controlled switching means, and (b) means to turn on said second switching means substantially simultaneously with said first switching means.
As compared to the Van Cleave arrangement discussed above, this arrangement has the advantages that it does not require any special arrangement or operating sequence of the motor windings in order to utilize the energy recovered by the capacitor, nor does it require the primary switching device to have bidirectional current carrying capabilities, as does the Van Cleave arrangement. Additionally, in the Van Cleave arrangement, the switching devices must be able to withstand, in their blocking condition, at least twice the maximum potential applied to the capacitor because of transformer action in the windings. Whilst this may not be a serious problem with the small stepper motors for which the Van Cleave arrangement is clearly intended, it becomes a serious limitation in larger motors.
As compared to the Ford patents discussed above, the values of the reactive components of the present applicant's arrangement do not limit the maximum current which can be drawn from the supply, nor the proportion of the active period of a phase winding during which current may be drawn if necessary. Essentially, the value of the capacitor in relation to the inductance of the associated winding determines the rates of current rise and fall which can be achieved in the winding, and the proportion of the active period of a phase winding during which it is necessary for current to be drawn from the supply. Under normal operation, the current to energize a winding is supplied from the capacitor, and current is only drawn from the supply in the latter part of the period during which the primary switching means is switched on, this current draw providing make-up for energy output to a load or dissipated by motor losses. It should be noted that the charge on the capacitor can be tapped by means of a suitable circuit so that the motor can operate also as a DC to DC up-converter, or so as to recover energy from a load under overrun conditions, or so as to provide regenerative braking of a load. Under such overrun or braking conditions the energy stored in the capacitor will be in excess of that required to maintain rotation of the motor and the excess may be recovered by drawing current from the capacitor when its potential exceeds a certain level.